阅读理解：

　　The Earth has a force that pulls things toward itself. We call this force gravity（引力）. This is something we love with all the time, and hardly ever think about it. But it is a most important factor（因素）in rocket（火箭）operation and must be overcome if we are to get anywhere in space, or off the ground at all.

　　Take the throwing of a ball as an example. The harder the ball is thrown, the faster and higher it will go. What is the secret? Its speed. If we could throw the ball hard enough it would go up and up and up forever and never come down. The speed at which it would have to be thrown to do this is known as escape speed. Of course, we cannot throw a ball hard enough because the speed required to escape completely from the Earth’s gravity is seven miles per second, or over twenty-five thousand miles per hour.

　　Once escape speed has been reached by a spaceship, no further power is needed. A rocket aimed at the Moon, for instance, will “:coast”（滑行）the rest of the way because the Earth’s gravity cannot then pull it back, and there is no air resistance in space to slow it down. This “coasting” is known as “free fall”. That does not mean the rocket is falling down towards the Earth but that it is travelling freely in space without the help of power, like a bicycle coasting downhill.

　　Free fall is an important feature of space travel: it would be impossible to carry enough oil to supply powered flight all the time.

1．What is the most important factor for a rocket to escape from the Earth?　　

　[　　]

A．To travel as fast as it can.

B．To overcome the Earth’s gravity.

C．To get away from the air resistance.

D．To have strong power in its operation.

2．How fast will a rocket escape completely from the Earth’s gravity?

　[　　]

A．Seventy miles per second.

B．Seven thousand miles per hour.

C．Twenty-five thousand miles per second.

D．Twenty-five thousand miles per hour.

3．As there is always the Earth’s gravitational pull, people ______.

[　　]

A．accept it without questioning it

B．never feel interested in its presence

C．hardly think about how to use it

D．often try to escape from it

4．When a bicycle “coasts” downhill, it ______.

　　　[　　]

A．runs along the hillside

B．does not need any further power

C．runs freely in all directions

D．does not meet with air resistance

5．It’s unnecessary to supply power for the flight in space all the time, because ______.

　　　[　　]

A．few spaceships complete their flight operations

B．the Earth’s gravity cannot be overcome

C．no further power is needed after the escape speed is reached

D．“free fall” takes the place of the Earth’s gravity